1. Field of the Invention
This invention relates to a method of processing a substrate of a printed circuit board or the like using a laser beam.
2. Related Background Art
Heretofore, through holes of printed circuit boards were formed by a mechanical boring process using a drill, and for forming small-diameter holes expensive super-hard drills having high mechanical strength have been used. Conductive material is precipitated in the holes formed with the drill.
FIGS. 1(a) and 1(b) illustrate an example such prior art methods of processing. Shown in FIG. 1(a) is a sectionnal view of a double-side glass epoxy substrate with a hole formed using drill 6, showing random irregularities of the hole wall surface and deviation of the upper and lower open end positions from each other (due to shooting of the drill). Shown in FIG. 1(b) is a conductor formed on the hole wall.
Since in this example the hole is formed mechanically using a drill, the following problems are encountered when the hole diameter is reduced.
1) The drill diameter is reduced, and therefore the mechanical strength of the drill edge and life of drill are reduced.
2) To obtain as long drill life as possible, the feed speed has to be reduced. Therefore, the processing time is increased.
3) The straightness and positional accuracy of the hole are inferior.
4) The irregularities of the hole wall are large with respect to the hole diameter, and therefore, it is difficult to form conductive material uniformly on the hole wall. Reliability with respect to various stresses is thus inferior.
There have been proposed method of forming holes having very small diameters using laser beams (as disclosed in, for instance, Japanese Laid-Open Patents 58-20390, 58-168489, 62-254117 and 62-289390).
Japanese Laid-Open Patent 62-254117 discloses a medical treatment apparatus for cutting a body portion by irradiation with a laser beam, particularly with two different laser beams from independent lasers (i.e., CO.sub.2 laser and YAG laser).
Japanese Laid-Open Patent 62-289390 a laser processing machine for irradiating the same place for processing with two laser beams having different wavelength. Particularly, a oscillation efficiency and high output long wavelength laser is used as main beam, and a high absorptivity short wavelength laser is used to provide a pre-pulse laser beam output. In this laser processing machine, optical means for making the optical axes of these two beams to be indentical is provided for irradiating the same place with the two beams. As the high output long wavelength laser are used CO.sub.2 lasers, CO.sub.2 lasers, YAG lasers, etc., and as the short wavelength laser are used excimer lasers, solid lasers of rubby or alexandlite, metal vapor lasers and like high recurring rate pulse lasers.